求大佬解惑

到底是哪的问题？

#include <bits/stdc++.h>
using namespace std;

int l,m;
int u,v;

int main(){
//  freopen("123.txt","r",stdin);
//  freopen("123.txt","w",stdout);
    scanf("%d%d",&l,&m);
    int cut=0;
    for(int i=1;i<=m;i++){
        scanf("%d%d",&u,&v);
        l-=v-u;
        cout<<l<<endl;
    }
    printf("%d",l);
    return 0;
}